1. Field of the Invention
The invention relates to a frequency-separator waveguide module with double circular polarization more particularly intended to serve as an antenna access module for a transmitter-receiver operating simultaneously in two frequency bands and with circular polarizations which are opposite for transmission and for reception.
2. State of the Art
This type of transmitter-receiver and, consequently, this type of module are especially intended to be used in systems transmitting and receiving at high bit rates via low-orbit satellites. The possibility of simultaneous transmission and reception with the same access point to a system means that it is possible to obtain high isolation between the transmission path and the reception path, at the antenna access point, and double circular polarization with a high degree of purity of polarization over a large frequency band. Right circular polarization for the transmission path and left circular polarization for the reception path are, for example, chosen. Cross-polarization of less than −25 dB, corresponding to an axial ratio of less than 1 dB, at the transmission access point and at the reception access point is, for example, sought.
A conventional approach for obtaining circular polarization from a linearly polarized field is shown diagrammatically in FIG. 1. Said approach combines an exciter 1 with a polarizer 2 made using waveguide technology. The exciter 1 separates a frequency band Tx used in transmission and a frequency band Rx used in reception. The polarizer 2 generates circular polarization, the handedness of which depends on the orientation of the electric field vector, as symbolized by the labels RCP and LCP, one assumed to correspond to right polarization and the other to left polarization.
A known waveguide component which makes it possible to produce such circular polarizations is a system with a central septum where steps produced on the septum border create a horizontal field which recombines with a vertical input field in order to produce circular polarization. In a known embodiment, shown schematically in FIG. 2, the polarizer 2 comprises two access points 3A, 3B made of waveguide with a rectangular cross section, symmetrically arranged with respect to a central plane of line XX′, which join each other at an end which is extended by a septum 4, in order to open out into a waveguide portion 5 with a square cross section where the septum is placed. The right or left circular polarization is obtained by the progressive creation of a horizontal electrical field vector, by the steps on the plate forming the septum 4 and the recombination of this horizontal vector with the vertical vector corresponding to the linear polarization of the access point 3A or 3B from which it comes. The two access points 3A and 3B therefore make it possible to produce two circular polarizations having orientations which are opposite for two different frequency bands at the access point 3C which constitutes the end of the portion 5 with a square cross section. The latter may possibly be fitted with a normal transition (not shown), making it possible to pass from a square section to a circular section, if necessary.
The separator 1 is combined with the polarizer 2 in order to separate the transmission Tx and reception Rx paths for each of the access points 3A and 3B. Provision is made to absorb, via a load, the band which is not useful at each of these access points 3A, 3B.
This is because, if the access points 3A and 3B are used alone, without a separator as envisaged above, there is a reflection of the frequency band which is not used at one access point, that is therefore of the band used for reception in the case of an access point used in transmission and vice versa. The consequence of these reflections in the direction of the septum results in mismatching of the polarizer. This is the reason for inserting a load, in this case assumed to be 50 ohms, in one arm and, for example, in an arm 6A parallel to the arm 7A at the access point 3A where the arm 7A is used for transmission, and the reason for inserting a similar load in the arm 6B parallel to the arm 7B at the access point 3B where the arm 7B is used for reception.
However, this solution has the drawback of being bulky because of the use of a separator with multiple arms for access. Furthermore, it is expensive since the components employed, such as the filters, the transitions and the septum, are awkward to produce and assemble.